1. Field of the Invention
This invention relates to a heat exchanger assembly and method of operation appropriate for use with a frost accumulating heat exchanger. More particularly, the present invention relates to a multi-row heat exchanger wherein the frost accumulates primarily on a single row and means are provided for routing refrigerant during defrost to that single row.
2. Prior Art
In many air conditioning and refrigeration applications a heat exchanger is used under conditions wherein water is deposited on the heat exchanger surfaces. For example, the outdoor heat exchanger of a heat pump operating in the heating mode serves as an evaporator absorbing heat energy from ambient air being circulated thereover. As the ambient air temperature is decreased its ability to hold water vapor is additionally decreased and excess condensed water vapor will be deposited on the heat exchange surface as water. If this surface is below freezing, ice will accumulate and the heat transfer efficiency between air and the heat exchanger will be diminished. In addition, if it is raining or snowing, this moisture may be drawn into the heat exchanger by its air handling apparatus or forced on to the heat exchanger by the wind.
In a cold room or other applications where an evaporator is operating below freezing to cool the air being supplied to the room a similar problem may occur. The reduction in temperature of the air being circulated over the heat exchanger below its dew point acts to condense out moisture which may freeze on the coil surfaces impeding heat transfer.
Most heat pump systems include means for eliminating frost from the coil surface. One of the most common means of defrost is to reverse the heat pump placing the heat pump system in the cooling mode wherein heat energy is discharged to the outdoor coil then serving as a condenser. Heat energy is supplied by the hot gas from the compressor being circulated to the outdoor heat exchanger wherein it serves to raise the temperature of the heat exchanger and to melt the frost accumulated thereon.
It has been found in multi-row heat exchangers that the frost buildup accumulates primarily on the first row of the coil. As used herein, the first row of the coil will mean that row which the ambient air flows over first as it enters the coil, the row that first reduces the temperature of said air. The condensate from the air as it is cooled collects primarily on the first row of the coil and allows the inner rows of the coil to be substantially unaffected by frost accumulation. As the frost accumulates it builds up on the first row of the coil not only effecting heat transfer between refrigerant flowing through the heat exchanger and air flowing thereover but actually may impede air flow between the heat transfer surfaces.
Under some frost conditions it has been found that the bottom rows of an inner row of the heat exchanger accumulates frost as well as the first row of the coil.
In order to effectively direct hot gaseous refrigerant to the location where the frost has accumulated the present invention provides for a heat exchanger assembly having the appropriate flow circuiting such that when the heat exchanger is in the defrost mode hot gaseous refrigerant is supplied directly to the coil areas having the frost accumulated thereon. The remaining coil areas substantially unaffected by frost are isolated such that heat energy is not used to heat the non-iced coil rows.